Abstract
We present AT2020mrf (SRGe J154754.2+443907), an extra-galactic (
z
= 0.1353) fast blue optical transient (FBOT) with a rise time of
t
g
,rise
= 3.7 days and a peak luminosity of
M
g
,peak
= ...−20.0. Its optical spectrum around peak shows a broad (
v
∼ 0.1
c
) emission feature on a blue continuum (
T
∼ 2 × 10
4
K), which bears a striking resemblance to AT2018cow. Its bright radio emission (
ν
L
ν
= 1.2 × 10
39
erg s
−1
;
ν
rest
= 7.4 GHz; 261 days) is similar to four other AT2018cow-like events, and can be explained by synchrotron radiation from the interaction between a sub-relativistic (≳0.07–0.08c) forward shock and a dense environment (
M
̇
≲
10
−
3
M
⊙
yr
−
1
for
v
w
= 10
3
km s
−1
). AT2020mrf occurs in a galaxy with
M
*
∼ 10
8
M
⊙
and specific star formation rate ∼10
−10
yr
−1
, supporting the idea that AT2018cow-like events are preferentially hosted by dwarf galaxies. The X-ray luminosity of AT2020mrf is the highest among FBOTs. At 35–37 days, SRG/eROSITA detected luminous (
L
X
∼ 2 × 10
43
erg s
−1
; 0.3–10 keV) X-ray emission. The X-ray spectral shape (
f
ν
∝
ν
−0.8
) and erratic intraday variability are reminiscent of AT2018cow, but the luminosity is a factor of ∼20 greater than AT2018cow. At 328 days, Chandra detected it at
L
X
∼ 10
42
erg s
−1
, which is >200 times more luminous than AT2018cow and CSS161010. At the same time, the X-ray emission remains variable on the timescale of ∼1 day. We show that a central engine, probably a millisecond magnetar or an accreting black hole, is required to power the explosion. We predict the rates at which events like AT2018cow and AT2020mrf will be detected by SRG and Einstein Probe.
Abstract
We present low-fRequency radio observations of a fast-rising blue optical transient (FBOT), AT 2018cow, with the upgraded Giant Metrewave Radio Telescope (uGMRT). Our observations span
t
= ...11–570 days post-explosion and a frequency range of 250–1450 MHz. The uGMRT light curves are best modeled as synchrotron emission from an inhomogeneous radio-emitting region expanding into an ionized medium. However, due to the lack of information on the source covering factor, which is a measure of the degree of inhomogeneity, we derive various parameters assuming the source covering factor to be unity. These parameters, hence, indicate limits on the actual values in an inhomogeneous model. We derive the lower limit of the shock radius to be
R
∼ (6.1−14.4) × 10
16
cm at
t
= 138−257 days post-explosion. We find that the fast-moving ejecta from the explosion are moving with velocity
v
> 0.2
c
up to
t
= 257 days post-explosion. The upper limits of the mass-loss rate of the progenitor are
∼ (4.1−1.7) × 10
−6
M
⊙
yr
−1
at (19.3−45.7) years before the explosion for a wind velocity
v
w
= 1000 km s
−1
. These
values are ∼ 100 times smaller than the previously reported mass-loss rate 2.2 years before the explosion, indicating an enhanced phase of the mass-loss event close to the end-of-life of the progenitor. Our results are in line with the speculation of the presence of a dense circumstellar shell in the vicinity of AT 2018cow from previous radio, ultra-violet, and optical observations.
ABSTRACT
We present extensive radio observations of a Type Ic supernova, ASASSN-16fp. Our data represent the lowest frequency observations of the supernova beyond 1000 d with a frequency range of ...0.33–25 GHz and a temporal range of ∼8–1136 d post-explosion. The observations are best represented by a model of synchrotron emission from a shocked circumstellar shell initially suppressed by synchrotron self-absorption. Assuming equipartition of energy between relativistic particles and magnetic fields, we estimate the velocity and radius of the blast wave to be $v$ ∼ 0.15c and r ∼ 3.4 × 1015 cm, respectively, at t0 ∼ 8 d post-explosion. We infer the total internal energy of the radio-emitting material evolves as E ∼ 0.37 × 1047 (t/t0)0.65 erg. We determine the mass-loss rate of the progenitor star to be $\dot{M} \sim (0.4\!-\!3.2) \times 10^{-5}\, \mathrm{M}_{\odot }\, \rm yr^{-1}$ at various epochs post-explosion, consistent with the mass-loss rate of Galactic Wolf–Rayet stars. The radio light curves and spectra show a signature of density enhancement in the circumstellar medium at a radius of ∼1.10 × 1016 cm from the explosion centre.
Radio Evolution of a Type IIb Supernova SN 2016gkg A. J., Nayana; Chandra, Poonam; Krishna, Anoop ...
Astrophysical journal/The Astrophysical journal,
08/2022, Letnik:
934, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
We present extensive radio monitoring of a Type IIb supernova (SN IIb), SN 2016gkg during
t
∼ 8−1429 days postexplosion at frequencies
ν
∼ 0.33−25 GHz. The detailed radio light curves and ...spectra are broadly consistent with self-absorbed synchrotron emission due to the interaction of the SN shock with the circumstellar medium. The model underpredicts the flux densities at
t
∼ 299 days postexplosion by a factor of 2, possibly indicating a density enhancement in the circumstellar medium due to a nonuniform mass loss from the progenitor. Assuming a wind velocity
v
w
∼ 200 km s
−1
, we estimate the mass-loss rate to be
M
̇
∼
(2.2, 3.6, 3.8, 12.6, 3.7, and 5.0) ×10
−6
M
⊙
yr
−1
during ∼8, 15, 25, 48, 87, and 115 yr, respectively, before the explosion. The shock wave from SN 2016gkg is expanding from
R
∼ 0.5 × 10
16
to 7 × 10
16
cm during
t
∼ 24−492 days postexplosion indicating a shock deceleration index,
m
∼ 0.8 (
R
∝
t
m
), and mean shock velocity
v
∼ 0.1
c
. The radio data are inconsistent with a free–free absorption model and higher shock velocities are in support of a relatively compact progenitor for SN 2016gkg.
Abstract
Very-high-energy (VHE; 100 GeV <
E
≤ 100 TeV) and high-energy (HE; 100 MeV <
E
≤ 100 GeV) gamma rays were observed from the symbiotic recurrent nova RS Ophiuchi (RS Oph) during its outburst ...in 2021 August by various observatories, such as the High Energy Stereoscopic System (HESS), Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and Fermi-Large Area Telescope (LAT). The models that have been explored so far tend to favor a hadronic scenario of particle acceleration over an alternative leptonic scenario. This paper explores a time-dependent lepto-hadronic scenario to explain the emission from the RS Oph source region. We have used simultaneous low-frequency radio data observed by various observatories along with the data provided by HESS, MAGIC, and Fermi-LAT to explain the multiwavelength spectral energy distributions corresponding to 4 days after the outburst. Our results show that a lepto-hadronic interpretation of the source not only explains the observed HE-VHE gamma-ray data but the corresponding model synchrotron component is also consistent with the first 4 days of low-radio-frequency data, indicating the presence of nonthermal radio emission at the initial stage of the nova outburst. We have also calculated the expected neutrino flux from the source region and discuss the possibility of detecting neutrinos.
Chandra’s Insights into SN 2023ixf Chandra, Poonam; Chevalier, Roger A.; Maeda, Keiichi ...
Astrophysical journal. Letters,
03/2024, Letnik:
963, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract We report Chandra-ACIS observations of supernova (SN) 2023ixf in M101 on day 13 and 86 since the explosion. The X-rays in both epochs are characterized by high-temperature plasma from the ...forward shocked region as a result of circumstellar interaction. We are able to constrain the absorption column density at both Chandra epochs, which is much larger than that due to the Galactic and host absorption column, and we attribute it to absorption by the circumstellar matter in the immediate vicinity of SN 2023ixf. Combining our column density measurements with the published measurement on day 4, we show that the column density declines as t −2 between day 4 to day 13 and then evolves as t −1 . The unabsorbed 0.3–10 keV luminosity evolves as t −1 during the Chandra epochs. On the Chandra first epoch observation, when the SN was 13 days old, we detect the Fe K α fluorescent line at 6.4 keV indicating presence of cold material in the vicinity of the supernova. The line is absent on day 86, consistent with the decreased column density by a factor of 7 between the two epochs. Our analysis indicates that during 10–1.5 yr before explosion, the progenitor was evolving with a constant mass-loss rate of 5.6 × 10 −4 M ⊙ yr −1 .
Abstract
We present early-phase panchromatic photometric and spectroscopic coverage spanning the far-ultraviolet to near-infrared regime of the nearest hydrogen-rich core-collapse supernova (SN) in ...the last 25 yr, SN 2023ixf. We observe early “flash” features in the optical spectra due to confined dense circumstellar material (CSM). We observe high-ionization absorption lines (Fe
ii
, Mg
ii
) in the ultraviolet spectra from very early on. We also observe a multipeaked emission profile of H
α
in the spectrum beginning at ∼16 days, which indicates ongoing interaction of the SN ejecta with a preexisting shell-shaped CSM having an inner radius of ∼75 au and an outer radius of ∼140 au. The shell-shaped CSM is likely a result of enhanced mass loss ∼35–65 yr before the explosion assuming a standard red supergiant wind. The UV spectra are dominated by multiple highly ionized narrow absorption and broad emission features from elements such as C, N, O, Si, Fe, and Ni. Based on early light-curve models of Type II SNe, we infer that the nearby dense CSM confined to 7 ± 3 × 10
14
cm (∼45 au) is a result of enhanced mass loss (10
−3.0±0.5
M
⊙
yr
−1
) two decades before the explosion.
We present new 0.6-10 GHz observations of the binary neutron star merger GW170817 covering the period up to 300 days post-merger, taken with the upgraded Karl G. Jansky Very Large Array, the ...Australia Telescope Compact Array, the Giant Metrewave Radio Telescope and the MeerKAT telescope. We use these data to precisely characterize the decay phase of the late-time radio light curve. We find that the temporal decay is consistent with a power-law slope of t−2.2, and that the transition between the power-law rise and decay is relatively sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated) outflow, but is instead the classic signature of a relativistic jet. This provides strong observational evidence that GW170817 produced a successful jet, and directly demonstrates the link between binary neutron star mergers and short-hard gamma-ray bursts. Using simple analytical arguments, we derive constraints on the geometry and the jet opening angle of GW170817. These results are consistent with those from our companion very long baseline interferometry paper, reporting superluminal motion in GW170817.
Abstract
We present optical
UBVRI
photometry and low-to-medium resolution spectroscopic observations of type Iax supernova SN 2020sck spanning −5.5 days to +67 days from maximum light in the
B-
band. ...From the photometric analysis we find Δ
m
B
(15) = 2.03 ± 0.05 mag and
M
B
= −17.81 ± 0.22 mag. Radiation diffusion model fit to the quasi-bolometric light curve indicates 0.13 ± 0.02
M
⊙
of
56
Ni and 0.34
M
⊙
of ejecta are synthesized in the explosion. Comparing the observed quasi-bolometric light curve with the angle-averaged bolometric light curve of a three-dimensional pure deflagration explosion of
M
ch
carbon-oxygen white dwarf, we find agreement with a model in which 0.16
M
⊙
of
56
Ni and 0.37
M
⊙
of ejecta is formed. By comparing the +1.4 days spectrum of SN 2020sck with synthetic spectrum generated using
SYN++
, we find absorption features due to C
ii
, C
iii
, and O
i
. These are unburned materials in the explosion and indicate a C–O white dwarf. One-dimensional radiative transfer modeling of the spectra with
TARDIS
shows higher density in the ejecta near the photosphere and a steep decrease in the outer layers with an ejecta composition dominated mostly by C, O, Si, Fe, and Ni. The star-formation rate of the host galaxy computed from the luminosity of the H
α
(
λ
6563) line is 0.09
M
⊙
yr
−1
, indicating a relatively young stellar environment.
Abstract
We present optical, radio, and X-ray observations of a rapidly evolving transient SN2019wxt (PS19hgw), discovered during the search for an electromagnetic counterpart to the ...gravitational-wave (GW) trigger S191213g. Although S191213g was not confirmed as a significant GW event in the off-line analysis of LIGO-Virgo data, SN2019wxt remained an interesting transient due to its peculiar nature. The optical/near-infrared (NIR) light curve of SN2019wxt displayed a double-peaked structure evolving rapidly in a manner analogous to currently known ultrastripped supernovae (USSNe) candidates. This double-peaked structure suggests the presence of an extended envelope around the progenitor, best modeled with two components: (i) early-time shock-cooling emission and (ii) late-time radioactive
56
Ni decay. We constrain the ejecta mass of SN2019wxt at
M
ej
≈ 0.20
M
⊙
, which indicates a significantly stripped progenitor that was possibly in a binary system. We also followed up SN2019wxt with long-term Chandra and Jansky Very Large Array observations spanning ∼260 days. We detected no definitive counterparts at the location of SN2019wxt in these long-term X-ray and radio observational campaigns. We establish the X-ray upper limit at 9.93 × 10
−17
erg cm
−2
s
−1
and detect an excess radio emission from the region of SN2019wxt. However, there is little evidence for SN1993J- or GW170817-like variability of the radio flux over the course of our observations. A substantial host-galaxy contribution to the measured radio flux is likely. The discovery and early-time peak capture of SN2019wxt in optical/NIR observations during EMGW follow-up observations highlight the need for dedicated early, multiband photometric observations to identify USSNe.
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